SETI: A Detectable Neutrino Signal?

byPaul GilsteronJune 23, 2009

Somehow I never thought of the IceCube neutrino telescope as a SETI instrument. Deployed in a series of 1,450 to 2,450 meters-deep holes in Antarctica and taking up over a cubic kilometer of ice, IceCube is fine-tuned to detect neutrinos. That makes it a useful tool for studying violent events like galactic collisions and the formation of quasars, providing insights into the early universe. But SETI?

Perhaps, says Zurab Silagadze (Novosibirsk State University), who notes that most SETI work in the past has focused on centimeter wavelength electromagnetic signals. Says Silagadze:

Here we question this old wisdom and argue that the muon collider, certainly in reach of modern day technology… provides a far more unique marker of civilizations like our own [type I in Kardashev’s classification… Muon colliders are accompanied by a very intense and collimated high-energy neutrino beam which can be readily detected even at astronomical distances.

Image: The IceCube array in the deep ice, with Eiffel Tower suggesting scale. The dark cylinder is the AMANDA detector, incorporated into IceCube. Credit: NSF.

Muons are elementary particles that, like all such particles, have a corresponding antiparticle of opposite charge. Because they have no known substructure, muons and antimuons offer interesting opportunities for a collider. Their advantage over protons is that the effective collision energy is about ten times higher than for proton beams with the same energy. Moreover, muons are much heavier than electrons and produce less synchrotron radiation. You get higher energy levels with a cheaper collider that is shorter in circumference. Here’s a short backgrounder from Physics World on this.

So it makes sense that, if we can get around formidable practical challenges, we’ll eventually want to develop a muon collider. So, presumably, would an extraterrestrial civilization. And indeed, Silagadze discusses the practical uses of a high-energy neutrino beam in, for example, the study of the inner structure of a planet, or the use of collimated neutrino beams for communications. A 1979 paper by Mieczyslaw Subotowicz went so far as to argue that advanced cultures might deliberately choose neutrino channels for interstellar communications to shut out immature emergent civilizations from the ongoing conversation.

For that matter, is it possible that neutrinos could be used to set interstellar time standards? Note the following from Silagadze’s paper, which places these ideas in the context of the Kardashev scale for measuring the growth of technological civilizations:

Neutrino SETI was also proposed earlier with somewhat different perspective… It was suggested that type II (which have captured all of the power from their host star) and type III civilizations, spread throughout the Galaxy, may require interstellar time standards to synchronize their clocks. It is argued that mono-energetic 45.6 GeV neutrino pulses… produced in a futuristic dedicated electron-positron collider of huge luminosity may provide such standards. If there is an extraterrestrial civilization of this type nearer than about 1 kpc using this synchronization method, the associated neutrinos can be detected by terrestrial neutrino telescopes with an effective volume of the order of km3 of water…

IceCube, anyone? The beauty of neutrino SETI is that it can readily run in the background of concurrent neutrino-based astrophysical studies. Thus keeping an eye out for possibly artificial high-energy neutrino signals produced in muon colliders light years away makes a certain degree of sense. Will it succeed? Silagadze quotes Cocconi and Morrison’s classic paper: “The probability of success is difficult to estimate: but if we never search, the chance of success is zero.”

Note that in a paper submitted in 1975 to Nature (not accepted)
“Can life exist in neutron stars?” I have suggested that this kind of
life based on nuclear forces could communicate via neutrinos.
This was before the electronic era and I have not electronic version
of the paper (yet).
A softer version of the paper has been published in
Origins of Life and Evolution of Biospheres (8, 33, 1977)
“A model for a non-chemical form of life: crystalline physiology.”
see http://www.springerlink.com/content/gl66u1411h17ml3l/

But… Muons have a mean lifetime of 2.2 µs. How could we possible accelerate them to 99.9…% the speed of light before they decay? Even at very high velocities they decay in seconds. I’m guessing they would somehow have to be produced *after* some particle has been sufficiency accelerated?

The main obstacle to conducting SETI with a neutrino detector is that those who operate IceCube and similar devices are not only not in the business of using their instruments for seeking out alien intelligences, but would probably either ignore or dismiss such signals as anything other than natural.

Perhaps I am wrong here because I am not directly involved with IceCube, but my experience with those astronomers who detect supernovae and their reactions to the idea of ETI using them as natural beacons to get our attention does not make me think the neutrino physicists will be much different.

Granted I would not want to see astronomers and others think that every unusual signal is a message from alien beings – pulsars coming to mind here – but there seems to be too little middle ground in this area. I know professional scientists are rightly worried about the stigma that the UFO has attached to SETI, but I am more concerned about what we may be missing because of our deliberate blinders when it comes to extraterrestrial life.

And of course when the day comes that we do have proof of ETI, everyone and their brother will be screaming that they knew it all along but were repressed by others and so forth.

I think that this would be extravagantly expensive even for very advanced civilization, because it confers simply no advantage to traditional photons.
Creating a detectable amount of neutrinos will always require orders of magnitude more energy than producing detectable amount of photons. So what is the advantage, even if the energy per se is not a hindrance to your supercivilisation ? You would still need a reason to use neutrinos instead of more practical photons. ( you don’t walk on your arms instead of legs just because you are potentially capable of doing so )
Secrecy ? You could build a planet sized antenna, and then send signals precisely to one single planet with a fraction of the cost. Or, you could use quantum encrypted broadband which is according to our primitive understanding of physics unbreakable in principle.
And neutrinos don’t confer better secrecy. They are as easily detected for you as for the eavesdroppers, no high tech required. Just a cubic kilometer of photomultipliers in ice. Not much more difficult than giant radio dishes.
So what ? Penetration through nebulae ? Better, but old-fashioned relay stations are orders of magnitude better.
Demonstration of technological superiority ?
Still better, but, There are many ways of doing that. Why particularly this one ?
Completely different technology, like civilization of deep ocean creatures that discovered high powered muon accelerators even before they discovered effective radio transmission ?
This is probably the best reason why a civilization might attempt to communicate via neutrinos I can imagine. Highly speculative, I admit, but there is no other way to find out than trying.

Our detectors have to be of km^3 order, but ET may have something better. All you need is a far denser “antenna”. They could then even weaken the signal to discriminate against lower-technology civilizations such as ourselves.

There was one SF novel (that I’m aware of) that featured the detection of an ETI neutrino signal, although in the end that point was left nicely ambiguous. It was “His Master’s Voice” by Stanislaw Lem.

ljk writes “Perhaps I am wrong here because I am not directly involved with IceCube, but my experience with those astronomers who detect supernovae and their reactions to the idea of ETI using them as natural beacons to get our attention does not make me think the neutrino physicists will be much different.”

I’m the anthethis of a UFO fan but ljk has well illustrated some established career focused scientists’ antipathy to what I’d term “a dangerous idea”. Sad to say, human nature remains constant through the generations. I think that there are zero other technological species in the Virgo sector, but it is arrogance, folly and closed mindedness not to look, especially in exotic manners yet unexplored.

Sorry for the a’ symbols. What was supposed to be a right arrow or yield symbol showed up as a’

It is interesting to see what percentage of muons would survive accelerating over a potential of 30 GeV/km to a relativistic kinetic energy of 1 TeV, according to the calculations below for a Muon Linac.

Surviving percentage of muons is 95.57 %, but the accelerator length would be 333 km

I went ahead and ran computations for a whole host of composite particles, charged Mesons and Charged Baryons as well as for the Tauon which required much higher length specific acceleration potentials.

For most of these particles, if a high enough length speciific acceleration potential can be obtained, perhaps a Linac is the easiest to utililize instead of a synchrotron, the reason being the very short life time of the particles. In order to accelerate the particles rapidly enough to the point where many of the particles would still survive due to time dilation effects, such a very high accelerating potential might as well be used in a Linac.

However, their are some possible candidates of short half live particle for synchrotrons and the muon and antimuon are two of them.

A 1979 paper by Mieczyslaw Subotowicz went so far as to argue that advanced cultures might deliberately choose neutrino channels for interstellar communications to shut out immature emergent civilizations from the ongoing conversation.

I guess this is possible, but a more likely explanation for a non-electromagnetic ETI communications system is simply that it’s more efficient in some way. If we ever locate an ETI (RF) beacon, I would not be at all surprised to find that the beacon’s signal contained instructions on how to access a more advanced means of communications. And depending on how advanced the ultimate interstellar communications technology is, there may be additional steps along the way (kind of like an extended bootstrapping process).

ABSTRACT. The transparent Sun is modeled as a spherically symmetric and centrally condensed gravitational lens using recent standard solar model (SSM) data. The Sun’s minimum focal length is computed to a refined accuracy of 23.5 ± 0.1 AU, just beyond the orbit of Uranus. The Sun creates a single image of a distant point source visible to observers inside this minimum focal length and to observers sufficiently removed from the line connecting the source through the Sun’s center. Regions of space are mapped where three images of a distant point source are created, along with their associated magnifications. Solar caustics, critical curves, and Einstein rings are computed and discussed. Extremely high gravitational lens magnifications exist for observers situated so that an angularly small, unlensed source appears near a three-image caustic. Types of radiation that might undergo significant solar lens magnifications, as they can traverse the core of the Sun, including neutrinos and gravitational radiation, are discussed.

So how does one control a particle that could pass through a block of lead one billion miles long like it isn’t even there?

Perhaps most life in the Universe lives on neutron stars, or floats like a balloon in Jovian atmospheres, or is made of silicon and lives deep underground, or exists as a form of plasma at the cores of suns. Such beings if they can think might think we are implausible and never bother to look for us.

I’m rereading Greg Egan’s “Diaspora” and when they’re in orbit around a life-bearing planet the Carter-Zimmerman polis uses passive neutrino sensing to scan the oceans below. They get high resolution via exciting certain neutrino sensitive isotopes so only the modest neutrino energies from Vega’s output cause them to transition. Quite clever really. If we can imagine such in fiction, then why not in reality if nothing forbids it physically?

Abstract: We report on a study of the anisotropy in the arrival direction of cosmic rays with a median energy per Cosmic Ray (CR) particle of ~ 14 TeV using data from the IceCube detector.

IceCube is a neutrino observatory at the geographical South Pole, when completed it will comprise 80 strings plus 6 additional strings for the low energy array Deep Core. The strings are deployed in the deep ice between 1,450 and 2,450 meters depth, each string containing 60 optical sensors.

The data used in this analysis were collected from April 2007 to March 2008 with 22 deployed strings. The data contain ~ 4.3 billion downward going muon events. A two-dimensional skymap is presented with an evidence of 0.06% large scale anisotropy. The energy dependence of this anisotropy at median energies per CR particle of 12 TeV and 126 TeV is also presented in this work.

This anisotropy could arise from a number of possible effects; it could further enhance the understanding of the structure of the galactic magnetic field and possible cosmic ray sources.

Abstract: Intense neutrino beams that accompany muon colliders can be used for interstellar communications. The presence of multi-TeV extraterrestrial muon collider at several light-years distance can be detected after one year run of IceCube type neutrino telescopes, if the neutrino beam is directed towards the Earth. This opens a new avenue in SETI: search for extraterrestrial muon colliders.

When I was a member of IceCube, I thought of this idea and actually took the time to do some feasibility studies. Unfortunately, it won’t work. The problem is that for the energies attainable by accelerators (GeV to TeV), the neutrino cross-section is still fairly small. Most neutrinos pass through any neutrino detector. It is simply impossible to produce a high enough flux of neutrinos to be detectable on a distant planet.

When I brought up this idea with my colleagues, the first thing someone asked was whether we could “see” the LHC. Even a very powerful particle accelerator on our own planet turns out to be invisible.

One question this raises is whether the process of scientific discovery would necessarily take place in a way analogous to the manner in which humans do it.

We make hypotheses, and design experiments to test them. Is this the only way to do it? For us, it may be so. But I wonder whether creatures whose ‘brains’ (for want of a better word) are built differently may have another way of progressing.

Perhaps they, like the ancient philosophers of Earth, just think really really hard about the problem, and they come up with a glorious unified theory without actually doing any experiments. Sure, I can’t actually imagine how they could do this, but who knows what they might be capable of if they’re really very clever, and perhaps conceptualize in a totally alien way.

I think the question of whether they could do this or not would depend on the nature of the universe. If the laws of physics are necessarily the way they are, then this kind of physics-by-deduction would be possible (at least in principle). If the laws of physics are contingent (or perhaps there’s a multiverse of varieties) then no-one – no matter how clever – could think their way to the answer.

Of course, there are other scenarios. Perhaps the progression of knowledge for ET follows a completely different path to ours. Again, I can’t really imagine too many major diversions from the path we took, but then I only have one example of an advanced technological civilization to play with! What I’m getting at is that certain experiments that we think are necessary might simply never occur to ET, either because they’ve worked out the answer without needing the experiment, or they’ve progressed in a completely different way.

SN 1987A: not close enoughExisting neutrino and gravitational-wave detectors can be used in concert to observe gravitational waves given off during a nearby supernova — say physicists in Italy.

Gravitational waves are vibrations of space–time predicted by the general theory of relativity. A number of experiments are trying to detect gravitational waves by measuring tiny changes in the separation of two masses that are expected to occur when the waves traverse a detector.

However, none have been successful so far and the most convincing evidence yet for gravitational waves is that the orbital period of the Hulse–Taylor binary star system is shrinking at the precise rate associated with the emission of gravitational waves.

With a little bit of luck, however, the first direct detection of the waves could happen if a supernova occurs in our own galaxy. Such a massive stellar explosion produces a vast amount of light and other radiation, which could help physicists narrow down their search to the precise moment that the gravitational waves reach Earth. This would be a great help in boosting the sensitivity of gravitational wave detectors.

Perhaps it would be very difficult to generate neutrinos detectable across interstellar distances, however, we have a very powerful neutrino generator in our backyard, namely, the sun. Would it be possible to modify existing neutrino emissions to carry information? It is known that matter can modify neutrino oscillations, i.e. the MSW Effect (though not sure how practical this would be for a comm system). Thoughts?

Would it be possible to modify existing neutrino emissions to carry information? It is known that matter can modify neutrino oscillations, i.e. the MSW Effect (though not sure how practical this would be for a comm system). Thoughts?

What an interesting concept. I don’t know how workable it might be, but I’m hoping others will have thoughts on the matter.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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